The nature of the changes in an organism produced by learning remains a mystery despite years of research on the problem. This project deals with the effect of pharmacologic and physical treatments in rats and mice upon different indices of short term memory. We are interested in discovering the reasons why registration and retrieval are so much more susceptible to drug effects than retention. Retention appears to be susceptible to drug action and physical treatments during the so-called consolidation phase of retention and much of our effort is devoted to an attempt to elucidate this process. Some specific aims at present are: improvement of short term memory tests for rats and mice; characterization of the conditional and unconditional stimuli which result in rapid associations and of environmental factors tending to facilitate or impair retention; a search for common properties of treatments which produce retrograde amnesia; localization of brain areas implicated in learning the same tasks; investigation of the role of known and putative neurohumoral systems in learning; attempts to influence biochemical mechanisms possibly involved in learning.